1. Field of the Invention
The present invention relates, in general, to microwave ovens using light wave heaters, such as halogen heaters capable of radiating high power light waves, in addition to magnetrons and, more particularly, to a heater cover used for shielding and protecting such a halogen heater from a cooking chamber in a microwave oven.
2. Description of the Prior Art
As well known to those skilled in the art, several types of cooking heaters, directly or indirectly using electric energy while cooking, have been proposed and used. An example of the conventional heaters is a microwave oven designed to use microwaves as heating energy while cooking. In such a microwave oven, a magnetron is electrically operated to generate microwaves and applies the microwaves to food in a cooking chamber, thus allowing the microwaves to cause molecular activity in the food. Such molecular activity in the food generates molecular kinetic energy, thus heating and cooking the food. Such microwave ovens are advantageous in that they have a simple construction and are convenient to a user while cooking, and easily and simply heat food in the cooking chamber. The microwave ovens are thus somewhat preferably used for some cooking applications, such as a thawing operation for frozen food or a heating operation for milk requiring to be heated to a desired temperature.
On the other hand, an electronic range, designed to use light waves in place of microwaves as heating energy while cooking, has been proposed as disclosed in U.S. Pat. No. 5,036,179. The above U.S. electronic range is designed to perform various desired cooking modes using a lamp capable of radiating visible rays and infrared rays. That is, this type of electronic range uses a lamp, wherein at least 90% of the radiation energy has a wavelength of not longer than 1 .mu.m. as a heat source. In said electronic range, both visible rays and infrared rays from the lamp are appropriately used, and it is possible to preferably heat a large quantity of food in a desired cooking mode.
The inventor of this invention proposed a microwave oven, designed to use such light waves in addition to microwaves as heating energy while cooking, in Korean Patent Application Nos. 97-60245 and 98-14106.
FIG. 1 shows the construction of a conventional microwave oven, using light waves in addition to microwaves as heating energy while cooking as proposed by the inventor of this invention. As shown in the drawing, the microwave oven has a cooking chamber 2 within an oven cavity 1, with two halogen heaters 10 installed on the top wall of the cavity 1. A heater box 14 covers the halogen heaters 10 at a position above the heaters 10, thus shielding and protecting the heaters 10 from surroundings. A heater cover 12 is installed at a position between the heaters 10 and the cooking chamber 2. In the drawing, the reference numeral 3 denotes a machine room for both a magnetron and a high voltage transformer.
The heater cover 12 is densely holed to have a plurality of perforations 12a through which both the visible rays and the infrared rays pass, radiated from the halogen heaters 10, to be introduced into the cooking chamber 2. The heater cover 12 has a collateral function of almost complete prevention of undesirable transmission of microwaves from the cooking chamber 2 to the halogen heaters 10.
In order to allow a smooth radiation of the light waves from the heaters 10 into the cooking chamber 2 while accomplishing such an almost complete prevention of undesirable transmission of microwaves from the chamber 2 to the halogen heaters 10, it is necessary to optimally design the profile, the size and the intervals of the perforations 12a. Of course, it may be preferable to make large-sized perforations 12a or to directly expose the heaters 10 to the upper portion of the chamber 2 so as to accomplish the smooth radiation of the light waves from the heaters 10 into the chamber 2. However, when the halogen heaters 10 are designed to be exceedingly exposed to the chamber 2 as described above, the heaters 10 may be easily affected by the microwaves from the chamber 2, thus being undesirably damaged or undesirably shortened in its expected life span. When the perforations 12a are designed to have an exceedingly small size, they reduce the amount of light waves effectively radiated from the heaters 10 into the chamber 2. In addition, such small-sized perforations 12amay cause the heater cover 12 and/or the heater box 14 to be easily damaged by heat.
FIG. 2 shows the construction of a conventional heater cover 12. As shown in the drawing, the conventional heater cover 12 has a plurality of perforations 12a. In the cover 12, the perforations 12a are arranged in a matrix with both a regular interval G between the rows L1, L2 and L3 of the perforations 12a and intervals a and b between the perforations 12a. In the conventional cover 12, the intervals G, a and b in addition to the size of the perforations 12a are not precisely set, but are roughly determined.
An example of conventional heater covers having such perforations may be referred to Japanese Patent Laid-open Publication No. Sho. 51-60,042. In the above Japanese heater cover, the size of perforations is set to 0.8 mm or less, while the interval between the perforation centers is set to 1.2 mm or less. The above dimensions finally set the interval between the edges of the perforations to 0.4 mm. However, it is almost impossible to form such perforations on a heater cover through a conventional physical process, such as a punching process. Therefore, a chemical process, such as an etching process, is used for forming such perforations. Such a chemical process of forming the perforations undesirably increases the production cost of heater covers in comparison with physical processes. In addition, the structure of the above Japanese heater cover is problematic as follows.
That is, the above heater cover 12 fails to have an optimal structure, which allows a smooth radiation of light waves from the halogen heaters 10 into the cooking chamber 2 while accomplishing the almost complete prevention of the undesirable transmission of microwaves from the chamber 2 to the halogen heaters 10. In other words, it is almost impossible to accomplish an optimal opening ratio of the heater cover 12 with the perforations 12a designed as shown in FIG. 2. Such perforations 12a reduce transmissivity of light waves through the heater cover 12, thus finally deteriorating thermal efficiency of the heaters 10. Particularly, in the case of high power halogen heaters, the heater cover 12 may be easily and thermally deformed or damaged.
FIG. 3 shows the construction of another type of conventional heater cover. In this heater cover, the perforations 12a are designed to be individually shaped in a rectangular profile. In the above heater cover 12, the length of each side of a rectangular perforation 12a is set to A. In addition, the interval between the rows L1, L2 and L3 of the perforations 12a, or the interval between the rectangular perforations 12a, is set to a'. The diagonal length of each rectangular perforation 12ais set to D.
When the arrangement of such rectangular perforations 12a is designed to have the shortest interval a' capable of maximizing the opening ratio of the heater cover 12, or when the interval a' remains the same as the interval a of the circular perforations 12a of FIG. 2, the heater cover 12 is problematic in that it fails to have a desired structural strength. Therefore, it is necessary for the heater cover 12, having such rectangular perforations 12a, to have an interval a' larger than that of the circular perforations. When the arrangement of the rectangular perforations 12a is designed as described above, a desired structural strength of the heater cover 12 is accomplished. However, such an arrangement of the rectangular perforations 12a reduces the opening ratio of the heater cover 12.
When the length A of each side of a rectangular perforation 12a is set to the diameter 2r of a circular perforation, the diagonal length D of the rectangular perforation 12a becomes larger than the diameter 2r of the circular perforation. In such a case, the heater cover 12 is seriously affected by the microwaves from the cooking chamber 2 and is undesirably reduced in structural strength. In order to allow the heater cover 12 having the rectangular perforations 12a to accomplish the same microwave shielding effect as that expected from the heater cover having the circular perforations, the diagonal length D of each rectangular perforation 12a may be set to the diameter 2r of each circular perforation. However, this undesirably reduces the opening ratio of the heater cover 12 and finally reduces thermal efficiency of the halogen heaters 10.
In this regard, it is necessary for manufacturers of such microwave ovens using halogen heaters 10 to design the heater cover 12 with an optimal opening ratio, an effective protection of the halogen lamps 10 from microwaves, and a desired structural strength. This object may be accomplished by optimally designing both the diameter of each perforation 12a of the heater cover and the intervals G, a and b of the perforations 12a.